The invention relates to the detection of random events in certain frequency domains, such as the detection of a fire. Embodiments of the invention to be described have improved discrimination against non-fire sources of varying radiation which might be confused with radiation emitted by fire, wherein the non-fire sources are characterized by repeating signal patterns.
In optical fire detectors which use flame flicker signals as a basis for the detection strategy, problems can arise with some types of bright sources which are readily confused with genuine flames. For instance, in the infrared band, radiant electric heaters send a continuous (DC) source of energy which normally falls outside the flicker band width of the flame detector unit. If some object happens to interrupt the light beam falling on the flame detector, the change in the signal level can be misinterpreted as a signal in the flicker band of the fire detector. In this case, the radiant heater could be considered to have been chopped by the interruption. Regular patterns of such interruptions can easily be the cause of false alarms.
Previously, methods have been worked out to analyze the putative fire signal to ensure that the signal meets certain statistical criteria which verify the randomness of the signal. For instance, U.S. Pat. No. 5,006,710 describes a method based on analysis of the turning points of the fire signal. By analyzing the amplitude of the signal between two turning points, a computation can be performed which distinguishes between a random signal (a property of most flames) and repetitive signals (a property of false signals). The computation is based on the amplitudes of the flicker peaks. A repetitive signal generates peak amplitudes of very similar height. The specified computation allows very different results to be generated when random peaks are used as inputs.
In practice, the performance of this amplitude analysis is not totally satisfactory. Testing with real false alarm sources show that the computation is easily fooled. For instance, if the repetitive signal consists of two or more peaks of differing heights, the algorithm will tend to call the signal random. Alternatively, if a flashing test lamp is held in front of a flame detector, the tester's involuntary muscle movement will cause amplitude modulation of the beam as it falls on the detector. This movement can cause the amplitude algorithm to misbehave.
This shortcoming can be overcome if the time between peak values, or turning points, rather than the amplitude between turning points is analyzed. A random signal has the time between turning points at random intervals. A repetitive signal repeats the time between turning points in a pattern which can be readily detected. Patterns repeat with a differing number of peaks. Three turning points is the shortest pattern. Longer patterns with 4, 5, 6 or more turning points can be described. The disclosed invention describes an improved apparatus and method of determining the existence of a randomly varying signal from a radiation source to permit the apparatus to communicate a warning indicating the presence of a fire.